1. Field of the Invention
The present invention relates to image forming apparatuses, such as printers, facsimile machines, and copying machines.
2. Description of the Related Art
Image forming apparatuses are known that include a plurality of first image carriers, an intermediate transfer member, a first image forming device, a second image carrier, and a second image forming device (e.g., Japanese Patent Application Laid-open No. 2006-201743). The first image carriers are a plurality of photosensitive elements that carry thereon either yellow (Y), magenta (M), or cyan (C) toner images. The intermediate transfer member is an intermediate transfer belt that sequentially receives the Y, M, and C toner images from the photosensitive elements. The first image forming device is a color-image forming device that transfers the toner images from the intermediate transfer belt to a recording sheet during secondary transfer. The second image carrier is a photosensitive element that carries thereon a black (B) toner image. The second image forming device is a black-image forming device that transfers the black toner image from the photosensitive element directly to the recording sheet.
The image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743 includes a belt member that is rotatably supported by a plurality of rollers. The belt member carries the recording sheet thereon to both a direct transfer position and a secondary transfer position. The direct transfer position is the position at which the black toner image is directly transferred onto the recording sheet. The secondary transfer position is the position at which the color toner image is transferred from the intermediate transfer belt onto the recording sheet during the secondary transfer. In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743, when the recording sheet passes through both the direct transfer position and the secondary transfer position by rotation of the belt member, the black image, which is transferred onto the recording sheet at the direct transfer position, and the color image, which is transferred onto the recording sheet at the secondary transfer position, are superimposed upon each other so that a full-color image is formed on the recording sheet. Carrying of the recording sheet by using the belt member suppresses any fluctuation that will occur along the recording-sheet conveying path between the direct transfer position and the secondary transfer position, which enables stable conveyance of the recording sheet between the direct transfer position and the secondary transfer position.
A technology is used in the field of image forming apparatuses that increases the life time of the photosensitive elements by applying a lubricant to the photosensitive elements. Some of the lubricant that has been applied to the respective photosensitive elements for Y, M, and C is transferred to the intermediate transfer belt and is further transferred from the intermediate transfer belt to the belt member. Some of the lubricant that has been applied to the photosensitive element for B is also transferred to the belt member.
The transcription efficiency depends on the relation between the coefficient of friction of a member from which an image is transferred (hereinafter, “image transferring member”) and the coefficient of friction of a member that receives the image (hereinafter, “image receiving member”). During the toner-image transfer process, in order to maintain the transcription efficiency at a high level, it is necessary to set the coefficient of friction of the image receiving member higher than the coefficient of friction of the image transferring member. Therefore, in the color-image forming device, the coefficient of friction of the intermediate transfer belt is set higher than the coefficient of friction of the photosensitive elements but lower than the coefficient of friction of the recording sheet. The coefficient of friction of the intermediate transfer belt decreases due to the lubricant transferred from the photosensitive elements. However, because the photosensitive elements are always coated with the lubricant and because the lubricant is transferred from the intermediate transfer belt to the recording sheet and the belt member, the coefficient of friction of the intermediate transfer belt cannot decrease to a value lower than the coefficient of friction of the photosensitive elements. Therefore, for the transfer from the photosensitive elements to the intermediate transfer belt, the transcription efficiency is always at a sufficiently high level. In the black-image forming device, the coefficient of friction of the photosensitive element is set lower than the coefficient of friction of the recording sheet to maintain the transcription efficiency at a sufficiently high level.
Image quality adjustment is also used in the field of image forming apparatuses for adjusting, through process control or the like, the image quality in accordance with predetermined conditions, for example, the conditions immediately after power-on and the conditions when a total number of copies reaches a predetermined value.
In the image forming apparatuses that have a similar configuration as those in the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743, an optical sensor is arranged, as a toner-image detecting unit, in the belt-member moving direction and downstream of both the direct transfer position and the secondary transfer position. The color-image forming device forms Y, M, and C toner patterns and the black-image forming device forms a black toner pattern. The Y, M, C, and B toner patterns are transferred onto the belt member and thus a test pattern that includes the Y, M, C, and B toner patterns is formed on the belt member. The optical sensor then detects the test pattern that is formed on the belt member. The image forming conditions are adjusted for the color-image forming device and for the black-image forming device in accordance with the detected result.
The coefficient of friction of the belt member is set higher than the coefficient of friction of the intermediate transfer belt in order to maintain the transcription efficiency of the intermediate transfer belt at a sufficiently high level.
However, the lubricant is transferred from the photosensitive element for B and the intermediate transfer belt to the belt member. Therefore, the coefficient of friction of the belt member may decrease, due to the lubricant on the outer surface of the belt member, to a value lower than the coefficient of friction of the intermediate transfer belt. If the coefficient of friction of the intermediate transfer belt is higher than the coefficient of friction of the belt member, the Y, M, and C toner patterns cannot be adequately transferred onto the belt member and the density of the toner patterns formed on the belt member decreases significantly relative to the density of the toner patterns before the transfer. This prevents correct image quality adjustment and leads to unstable image formation.